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The workshop upload system should be more specific about what the issue is, instead of just giving a generic error. I had the same error when I tried adding a thumbnail picture. The error was gone when I putted it in the right folder (the mod one). Oh, and don't forget to click on fetch info first when you update, else it'll upload it as a new mod.
 
The workshop upload system should be more specific about what the issue is, instead of just giving a generic error. I had the same error when I tried adding a thumbnail picture. The error was gone when I putted it in the right folder (the mod one). Oh, and don't forget to click on fetch info first when you update, else it'll upload it as a new mod.
Which folder do you put the thumbnail in? that seemed to be my problem, once I removed it the upload worked.